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The possible role of catecholamines upon the nature of myocardial response to regional and global left ventricular ischaemia was investigated. Global ischaemia was accomplished by temporary occlusion of the left main coronary artery and regional ischaemia by temporary ligation of the left anterior descending coronary artery. Isometric force of contraction was measured with strain gauge arches, regional blood flow by a thermistor technique and intracellular NADH redox level by a fluorometric technique. These measurements were made simultaneously in two areas on the left ventricular myocardium: one immediately below the bifurcation of the LAD and the other immediately below the circumflex coronary artery. Following LAD occlusion a variable inotropic response was observed in the ischaemic area. A decrease in contractile tension was found in 19 of the dogs (44%), no change in 23% and an increase was found in 33%. Global ischaemia invariably resulted in a decrease in contractile tension. NADH redox state was markedly higher during global than during regional ischaemia. Propranolol administration blocked the positive inotropic response to regional ischaemia. It is concluded that endogenous catecholamine release may be responsible for increased contractile force during regional ischaemia. This response is apparently limited by the oxygen availability to the tissue.